The present invention is concerned with the forming of metal patterns for microelectronics applications, and more especially with "lift-off" processes.
In such processes, metal is deposited over a photoresist image which is subsequently dissolved away by immersion in a suitable solvent thereby lifting-off unwanted metal. Because the line width of the metal is determined by photolithography, lift-off patterning offers superior control of fine line widths to that obtainable by metal etch processes. Lift-off techniques using both single and bi-level photoresist layers for metal deposited both by conventional sputtering and by evaporation have been reported--see, for example, Tom Batchelder, "A Simple Metal Lift-Off Process", Solid State Technology, February 1982 pp 111-114, and J. M. Frary and P. Seese "Lift-Off Techniques for Fine Line Metal Patterning", Semiconductor International, December 1981, pp 72-88.
"Step coverage" is an important factor in such patterning techniques. In general, in the fabrication of integrated circuits, it will be necessary to deposit metal patterns on a substrate having topographical features such as steps, ridges and so forth due to the prior formation of integrated circuit structures. Step coverage refers to the ability of the fabrication process to provide satisfactory continuity of metal over such topographical features, and it is well known that the deposition processes referred to above result in a lower metal thickness on the side walls of such features. The requirements of satisfactory step coverage and satisfactory lift-off are conflicting since the lift-off mask should defeat metal step coverage in the mask openings, so that there is a weakness or discontinuity between the metal deposited directly on the substrate through the mask openings, and that which is to be lifted off the masked areas when the photoresist mask is dissolved.
Bias sputtering (in which an electrostatic potential is applied to the substrate during the sputtering process) provides an improvement in step coverage and has been successfully used with metal etch processes but has hitherto proved unsuitable for use in lift-off techniques for the reasons indicated above.